Running a stage 2 procharger @9psi. Not sure of hp as I have not been on a dyno yet. My car has 3.73s now and 1st and 2nd gears are absolutely usless as soon as boost hits. Also spins at the top of third a little. Definitely looking to go to a 3.55 or 3.31 gear this year.

It's probably safe to assume that you're having grip issues in anything below 4th gear, so by putting in a more aggressive final drive you won't see any better acceleration in those gears, and will probably just spin even more. The advantage in acceleration will come in the higher gears (4-6). Don't worry about if you shift into 3rd before 60 as it is a pretty arbitrary number, unless you are doing testing for MotorTrend. The more important number is the trap speed in the quarter mile, which you will more than likely be in 4th for either the 3.31 or 3.73 gears. I've included a link to a gear ratio calculator to play around with.

As far as putting a shorter gear in just to put on a larger diameter wheel, you would be spending money to spend money, as they would more or less counteract each other. There can be an argument to be made if you plan to spend a lot of time at the drag strip though.

It's probably safe to assume that you're having grip issues in anything below 4th gear, so by putting in a more aggressive final drive you won't see any better acceleration in those gears, and will probably just spin even more. The advantage in acceleration will come in the higher gears (4-6). Don't worry about if you shift into 3rd before 60 as it is a pretty arbitrary number, unless you are doing testing for MotorTrend. The more important number is the trap speed in the quarter mile, which you will more than likely be in 4th for either the 3.31 or 3.73 gears. I've included a link to a gear ratio calculator to play around with.

As far as putting a shorter gear in just to put on a larger diameter wheel, you would be spending money to spend money, as they would more or less counteract each other. There can be an argument to be made if you plan to spend a lot of time at the drag strip though.

Well I was gonna go 3.73. And if it was too much then go slightly bigger tire as a fine tune to turn it down some with more grip and diameter.

Also the bigger diameter tire compensates for top end speed with a shorter gear.
However wider rears will affect the turning some I gather?

Yes, these Mustang's have a habit of understeering at the limit when the rears are wider than the front. Unless you use the car for Autocross or road coarse duty, then you probably won't notice much of a difference since you shouldn't be going 10/10th's on public roads anyway.

Fireball241, I'll tell you a little secret. Forget about the rear end gears. 3.31is close enough. The gears that must be changed are the gears in your TRANNY. Back in the day I learned this secret from "oval racing." Check with the fellas at "Pro Motion Performance." Not to get to crazy, but here is a sampling of what I am talking about. Also, this is what I had to do to get excellent 60 footers and beautiful launches into the Twilight Zone. To determine any carís overall First-gear ratio, multiply the transmissionís First-gear ratio by its rear-gear ratio. If your Mustang has a 3.35 First gear and a 3.55 rear gear, the overall First-gear ratio is 3.35 x 3.55 = 11.89. Anything greater than 9.5 could be considered overkill for the street. Ever driven a Mustang with a 3.35 First-gear trans and a 3.73 rear? First gear is virtually useless, since the overall ratio is 12.49. This is why a 2.95 First-gear T5 or the Tremec 600 2.87 trans is far more desirable if your plans include deep rear gears.

Fireball241, I'll tell you a little secret. Forget about the rear end gears. 3.31is close enough. The gears that must be changed are the gears in your TRANNY. Back in the day I learned this secret from "oval racing." Check with the fellas at "Pro Motion Performance." Not to get to crazy, but here is a sampling of what I am talking about. Also, this is what I had to do to get excellent 60 footers and beautiful launches into the Twilight Zone. To determine any carís overall First-gear ratio, multiply the transmissionís First-gear ratio by its rear-gear ratio. If your Mustang has a 3.35 First gear and a 3.55 rear gear, the overall First-gear ratio is 3.35 x 3.55 = 11.89. Anything greater than 9.5 could be considered overkill for the street. Ever driven a Mustang with a 3.35 First-gear trans and a 3.73 rear? First gear is virtually useless, since the overall ratio is 12.49. This is why a 2.95 First-gear T5 or the Tremec 600 2.87 trans is far more desirable if your plans include deep rear gears.

Great stuff. I am curious about the comparison car mentioned above and the transmission it has.

However a 3.73 may expedite the upgrade of my stock transmission especially with bigger tires

Fireball, you are scaring me. Why, because you sound a lot like my thinking. Yes, the "taller" tires will cut down your rear gear ratio, but this will give you less "rollout" and effect your ET. Don't change your tire diameter before you change any gears. Tire size can always be adjusted. If you make "Transmission" gear changes, you just might get by without spending money on changing your "rear ends" gear ratio. Gearing and tire size has always been a "dark art." I changed the T-5 "tranny" gear set in my 1987 Foxbody with a 331 stroker, and it worked out perfect. Yup, I started out with 373s and worked my way down to 325s, and that was the right combination for "this" car. I am running NITTO NT555R P275/40R17, street/strip presently. Come summer, I am mounting Mickey Thompson Radials on 15" rims, strictly for competition. Gotta love the bigger sidewall. Take your time, and success will come to you. It only took me 60 years to get racing down to a fine point. By the way, you outline a very nice set up in your first post. If you leave everything the same, I would vote for 355 rear end gears, and have some fun.